1. Field of the Invention
The invention concerns a method for weighing items moving over a weighing plate in succession, particularly letters standing on edge. The method is suitable for users of mail processing systems with a dynamic scale and a postage-calculating franking machine, or with a dynamic postage calculation scale and a franking machines.
2. Description of the Prior Art
Mail processing systems typically include a mail item sorting (separating) device (see DE 27 17 721 A1 and DE 37 31 525 C2).
Such mail processing systems normally are described for mixed mail. This means a load range of 3 to 1000 grams, a load thickness between 0.5 and 20 millimeters and load lengths between 100 and 353 millimeters.
A letter sorting device is known (see DE 199 12 807 A1) that is equipped with means for determination of the letter dimensions. The data so determined are conducted to a following device such as a scale and/or franking machine (see also EP 1 189 041 A1).
In modern mail handling systems the scale is designed as a dynamic scale, to achieve faster throughput, as described, for example, in DE 198 33 767 C2 and EP 0 974 819 A2. The scale has a weighing plate with the base thereof formed by the transport belt of a transport device. The weighing plate has a rear wall for guidance of the mail item. The mail item is weighed during the transport. The weighing plate is executed with a lightweight construction such that it is bending-resistant and distortion-resistant and is coupled to a weighing cell at the approximate location of the center of gravity of the combination of the weighing plate and a mail item with the maximum allowable weight and the largest allowable dimensions arranged centrally on the weighing plate. The weighing cell is provided so that the maximal travel (excursion) is in the micrometer range. The length of the weighing plate is correspondingly adapted to the largest mail item length to be processed. This means that the distance between a long mail item and the next following mail item is at least equal to the weighing plate length plus the length for the necessary measurement time.
An arrangement for communication between stations of a mail processing machine is known (see EP 0 875 864 A2, in particular FIGS. 2b and 3b) in which a dynamic scale is connected according to the interface via data cables with each of a preceding automatic feed and a following franking machine. In detail, the scale has a controller, emergency stop logic, two interfaces and associated connection contact means. Inside the scale, the controller is connected with the emergency stop logic.
Furthermore, a method and an arrangement for determination of a weight with a dynamic scale are known from EP 1 014 052 A1. The arrangement has a transport device, a weighing plate, a weighing cell, sensors for the mail item intake and discharge as well as an electronic control unit. The control unit serves for control of the transport device as well as for evaluation of the measurement values transmitted by the weighing cell, for measurement value correction using determined parameters, and for output of a corrected weight value to a franking machine via an interface. According to the associated method, selection of a mid-range measurement value ensues for comparison with at least one overload limit value for testing to insure the absence of an overload condition and a decision parameter is formed for comparison with at least one deactivation criterion with regard for testing the validity of the measurement values transmitted from the weighing cell. In this method, the measuring time begins when the trailing edge of the postal item passes the input sensor, and the measurement time ends when the trailing edge passes the output sensor.
A method for controlling a dynamic scale is known (see EP 1 014 050 A1) that can process mixed mail items with different sizes and thicknesses and that can be operated in at least two operating modes, the mail items subsequently passing through a further processing station. In a manner analogous to the solution cited above, a transport belt driven by a motor, the transport belt simultaneously forming the floor of the weighing plate, serves for transport of the mail items. A controller for rotation speed regulation is provided for the motor. In a dynamic operating mode, when a mail piece is located in the intake of the scale, independent of its format the mail piece is conveyed with a transport speed (set with a default value) before the beginning of a first measurement time range.
In the first measurement time range, the rotation speed regulation by the controller of the scale is deactivated and an unregulated pulse voltage is applied to the direct current motor so that a dynamic weight measurement ensues with deactivated rotation speed regulation. In the first measurement time range, the transport speed is varied dependent on the weight of the mail piece by interaction of the direct current motor with a tension device for a defined setting of the belt tension of the transport belt, and with a vibration-damped design of the transport device of the scale. Thus in the case of dynamic weighing, heavy mail pieces are automatically transported slower and consequently the available measurement time is increased. The evaluation of the measurements ensues in the controller of the scale and requires a calculation time and a reaction time. The leading edge of the mail piece exits the weighing plate after the measurement time has elapsed. If after the elapse of the measurement time, an algorithm for error detection detects an error, the scale is automatically switched by the controller into a reweighing mode. The largest format to be processed is taken into account in the selected measurements of the weighing plate. The selected transport speed of the belt corresponds to the transport speed in the subsequent franking machine, but the set intervals from mail piece-to-mail piece reduce the cycle capacity of the scale to approximately two-thirds of the maximum cycle capacity of the franking machine. Experience has shown that, under these conditions, a re-measurement (requiring transport interruption and return movement of the mail piece) is seldom necessary.
When a mail piece is located in the exit region outlet of the scale, activation of the rotation speed regulation ensues again for the motor.
After the weighed mail piece has exited the scale, the subsequent mail piece is conducted onto the weighing plate. This means that the minimum separation between two successive mail pieces amounts to more than one weighing plate length; that is particularly disadvantageous in the case of short, light letters. However, significantly smaller separations between the mail pieces are allowed for the mail item processing in the franking machine.